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Date: Fri, 24 Jun 2011 16:21:38 -0500
From: Tycho Andersen <tycho@tycho.ws>
To: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Interpreting Left to right?
References: <BANLkTimcPAjQP4JJk=OAbrkLFfd2AtndwQ@mail.gmail.com> <iu1aaq$rni$1@dough.gmane.org> <4E043953.5080705@stoneleaf.us> <20110624195300.GJ6075@point.cs.wisc.edu> <4E04EFD4.6050309@stoneleaf.us> <20110624200618.GK6075@point.cs.wisc.edu> <iu2u0a$hre$1@dough.gmane.org>
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On Fri, Jun 24, 2011 at 05:02:00PM -0400, Terry Reedy wrote:
> On 6/24/2011 4:06 PM, Tycho Andersen wrote:
> 
> >tmp = {}
> >x['huh'] = tmp # NameEror!
> >
> >That is, the right hand sides of assignments are evaluated before the
> >left hand sides. That is (somehow?) not the case here.
> 
> You are parsing "a = b = c" as "a = (b = c)" which works in a
> language in which assignment is an expression, but does not work in
> Python where assignment is a statement. You have to parse it more as
> "(a = b) = c" but that does not work since then the first '=' is not
> what it seems. It is more like "(both a and b) = c". Perhaps best to
> expand "a = b = c" to "a = c; b = c" and see the first as an
> abbreviation thereof -- just delete the 'c;'.
> 
> If I have ever used this sort of multiple assignment, it has been
> for simple unambiguous things like "a = b = 0".

Ah, the point about the grammar is what I was missing. Thanks a bunch!

\t